REVAMP core to FIX #20 reuse of instanciated plugin digesters

HASHING REVAMP

Had to revamp to solve the bug in #20 about reusing instanciated plugin-digesters.

In the previous version, each "digester" was a "convoluted' 2-tuple (digester-instance, final-hash-func). To solve the reuse bug without refactoring it would require to re-initialize the plugins for each input.

In this PR registered digesters are actually factory_functions(fsize: int) that will create a digester class with just 2 methods:

update(bytes)
hexdigest() -> str # lower

NOTE: the factory-function takes fsize as its argument, - this is useful for git-digesters, to avoid always slurping files with known size; that is also handy for for URL-resources; but as bargain, all other digesters must use a "special" factory just to ignore the fsize arg.

MODULES REVAMP

As explained in #20, enabling plugins in Travis across PY-versions revealed structural module issues. In general it is easier if module-names do not shadow their package-name, and using relative imports is helpfull. So I had to move:

omniparse.omniparse.py --> omniparse.init.py
and move project coords --> omniparse._version.py.

Other changes

(most changes are in commit e15bef5552, impossible to separate, sorry)

FIX plugins - multiple problems were preventing them from running - added travis TCs to detect them.
Add -x family option to exclude families (that was easy) :-).
Implement inclusion/exclusion logic within a class that avoids needless instantiation of excluded digesters.
Do not git-slurp if URL provide Content-Length.
Avoid some top-level imports, to speed up cmd-line launch for help-msg.
FIX: was comparing hash-matches case-sensitively.
FIX: Stop printing this ugly message about plugins-failed-to-load, even if not installed!.
Centrally ensure all hashes are str/lower.
TravisCI on DEV for PY35+. PY36+; many things change in the future.
Add TCs to prove the errors found.

TODO:

We can move main-code into own __main__.py file, and do the trick I described in #10 with 2 cmds (oh, & omnihash).
Restructure a bit functions, grouping i.e. digester-setup functions together.
Unfortunately, PY36+ fails (allowed failure in Travis), so FIX this.
Retrofit CRCs as plugin.

Miserlou / omnihash